The invention relates to optical fluoride crystals, and particularly to optical fluoride crystals such as calcium fluoride, which have high transmission levels to below 200 nm light, such as produced by excimer lasers. In particular the invention relates to making optical fluoride crystals with improved transmission surfaces. The invention relates to the elimination of mid-spatial frequency roughness 1-1000 xcexcm spatial wavelengths and high-spatial frequency less than 1 xcexcm spatial wavelengths from optical fluoride crystal surfaces.
Applications of colloidal suspensions for polishing materials has become an exceedingly critical aspect of the final part formation of optical element components and blanks thereof. Silica and alumina colloids are formed through various techniques and typically require expensive precursor materials in order to ensure the highest purity products. Solutions are stabilized with buffer systems to pH and solids loading values that result in optimal surface finish attainment. Particle size distribution can be adjusted to control the final surface finish, as well as the ability to clean residue abrasive particles from workpiece surfaces after processing.
The level of final optical transmission surfaces currently available for optical fluoride crystals is not good enough for optical fluoride crystalline laser components and optical lithography elements.
The invention includes a method of making a wavelength xcex less than 200 nm optical fluoride crystal with the method including providing a fluoride crystal preform having a first and a second initial finished optical transmission surfaces having a xe2x89xa620 angstrom RMS surface roughness with a plurality of initial finished mid-spatial frequency roughness 1-1000 xcexcm spatial wavelengths and initial finished high-spatial frequency roughness less than 1 xcexcm spatial wavelengths and the initial finished fluoride preform having a low initial finish xcex less than 200 nm transmission LT (%/cm). The method includes providing a final polishing mid-spatial frequency and high-spatial frequency spatial wavelength removing colloidal particle solution having a pHxe2x89xa79 and a plurality of colloidal particles and final polishing the initial finished surfaces with the mid-spatial frequency and high-spatial frequency spatial wavelength removing solution into first and second final polished calcium fluoride optical transmission surfaces with the mid-spatial frequency and high-spatial frequency spatial wavelength removing solution removing the initial finished spatial frequency spatial wavelengths to provide a final finished optical fluoride crystal having final finish high optical transmission surfaces free of the mid-spatial frequency roughness 1-1000 xcexcm spatial wavelengths and the high-spatial frequency roughness less than 1 xcexcm spatial wavelengths with a xcex less than 200 nm high transmission HT (%/cm), with HT greater than LT, and transmitting a final use wavelength xcex less than 200 nm light through the final finish high optical transmission surfaces.
The invention includes a method of making a  less than 200 nm light transmitting optical fluoride crystal blank for transmitting less than 200 m light at a fluence less than 20 J/cm2. The method includes providing an optical fluoride crystal preform having a first and second initial finished parallel flat optical transmission surfaces having a xe2x89xa650 angstrom RMS surface roughness with an initial finished mid-spatial frequency (1-1000 xcexcm spatial wavelength) roughness and an initial finished high-spatial frequency ( less than 1 xcexcm spatial wavelength) roughness, with the initial finished preform having a low initial finish xcex less than 200 nm transmission LT (%/cm). The method includes providing a final surface processing colloidal non-friable spherical abrasive particle solution having a pHxe2x89xa79 and a plurality of colloidal non-friable spherical abrasive particles which have a mean particle size in the range from 20 to 300 nm. The method includes final polishing the initial finished surfaces with the final surface processing colloidal particle solution into a final polished optical transmission surface with the final polishing final surface processing colloidal particle solution removing the initial finished mid-spatial frequency roughness spatial wavelengths of 1-1000 xcexcm and the initial finished high-spatial frequency roughness spatial wavelengths less than 1 xcexcm to provide a final finished optical fluoride crystal blank having a final finish high optical transmission surface with a final finish xcex less than 200 nm high transmission HT (%/cm) with HT greater than LT.
The invention includes a method of making an ultraviolet xcex less than 200 nm qualified optical fluoride crystal qualified for use at a wavelength xcex less than 200 nm, and preferably at a fluence less than 20 J/cm2 at the less than 200 nm wavelength xcex. The method comprises providing an optical fluoride crystal preform having a first initial finished flat optical transmission surface with a xe2x89xa650 angstrom RMS surface roughness with initial finished mid-spatial frequency roughness with 1-1000 xcexcm spatial wavelengths and initial finished high-spatial frequency roughness with  less than 1 xcexcm spatial wavelengths, and the preform having a low initial finish xcex less than 200 nm transmission LT (%/cm). The method includes providing a final polishing mid-spatial frequency removing colloidal spherical abrasive particle solution, with the mid-spatial frequency removing solution having a pHxe2x89xa79, a plurality of colloidal particles having a mean particle size in the range from 20 to 300 nm, preferably with the particles comprised of SiO2. The method includes final polishing the initial finished surface with the mid and high spatial frequency removing solution into a final polished optical transmission surface with the mid-spatial frequency and high-spatial frequency removing solution removing the initial finished mid-spatial frequency roughness 1-1000 xcexcm spatial wavelengths and the initial finished high-spatial frequency roughness less than 1 xcexcm spatial wavelengths to provide a final finished optical fluoride crystal having a final finish high optical transmission surface with a final finish xcex less than 200 nm high transmission HT (%/cm) with HT greater than LT and transmitting a final use wavelength xcex less than 200 nm light beam with a fluence less than 20 J/cm2 through the final finish high optical transmission surfaces to provide a qualifying optical transmission measurement for said wavelength xcex less than 200 nm.
The invention includes a method of making a wavelength xcex less than 200 nm optical calcium fluoride crystal for use at fluences less than 20 J/cm2. The method includes providing a calcium fluoride crystal preform having a first and second initial finished flat optical transmission surfaces which have xe2x89xa650 angstrom RMS surface roughness with an initial finished mid-spatial frequency roughness 1-1000 xcexcm spatial wavelengths and an initial finished high-spatial frequency roughness less than 1 xcexcm spatial wavelengths, and the initial finished calcium fluoride preform having a low initial finish xcex less than 200 nm transmission LT (%/cm). The method includes providing a final polishing mid-spatial frequency and high-spatial frequency removing colloidal SiO2 particle solution having a pHxe2x89xa79 and a plurality of colloidal SiO2 particles with a mean particle size in the range from 20 to 300 nm. The method includes final polishing the initial finished surfaces with the mid-spatial frequency and high-spatial frequency removing solution into first and second final polished calcium fluoride optical transmission surfaces with the spatial frequency removing solution removing the initial finished spatial frequency wavelengths to provide a final finished optical calcium fluoride crystal having final finish high optical transmission surfaces with a xcex less than 200 nm high transmission HT (%/cm) with HT greater than LT. The method includes transmitting a final use wavelength xcex less than 200 nm light beam with a fluence less than 20 J/cm2 through the final finish high optical transmission surfaces to provide a qualifying optical transmission measurement for the xcexL less than 200 nm wavelength.
The invention includes a method of making a wavelength xcex less than 200 nm optical calcium fluoride crystal with the method including providing a calcium fluoride crystal preform having a first and a second initial finished optical transmission surfaces having a xe2x89xa620 angstrom RMS surface roughness with a plurality of initial finished mid-spatial frequency roughness 1-1000 xcexcm spatial wavelengths and initial finished high-spatial frequency roughness less than 1 xcexcm spatial wavelengths and the initial finished calcium fluoride preform having a low initial finish xcex less than 200 nm transmission LT (%/cm). The method includes providing a final polishing mid-spatial frequency and high-spatial frequency spatial wavelength removing colloidal particle solution having a pHxe2x89xa79 and a plurality of colloidal particles and final polishing the initial finished surfaces with the mid-spatial frequency and high-spatial frequency spatial wavelength removing solution into first and second final polished calcium fluoride optical transmission surfaces with the mid-spatial frequency and high-spatial frequency spatial wavelength removing solution removing the initial finished spatial frequency spatial wavelengths to provide a final finished optical calcium fluoride crystal having final finish high optical transmission surfaces free of the mid-spatial frequency roughness 1-1000 xcexcm spatial wavelengths and the high-spatial frequency roughness less than 1 xcexcm spatial wavelengths with a xcex less than 200 nm high transmission HT (%/cm), with HT greater than LT, and transmitting a final use wavelength xcex less than 200 nm light through the final finish high optical transmission surfaces.